Gas turbine bucket with impingement cooled platform

ABSTRACT

In a turbine bucket having an airfoil portion and a root portion, with a substantially planar platform at an interface between the airfoil portion and root portion, a platform cooling arrangement including at least one bore in the root portion and at least one impingement cooling tube seated in the bore, the tube extending beyond the bore with an outlet in close proximity to a targeted area on an underside of the platform.

BACKGROUND OF THE INVENTION

[0001] This invention relates to the cooling of turbine buckets and,specifically, to the cooling of the platform region of the bucket, atthe base of the airfoil.

BRIEF SUMMARY OF THE INVENTION

[0002] Stage one and stage two buckets in gas turbines experience hightemperatures and stress, creating low cycle fatigue life concerns,particularly at the leading edge fillet where the airfoil joins theplatform at the forward portion of the pressure side of the airfoil.

[0003] In aircraft engines, the bucket platforms are cooled by airentering the cavity below the platform, but this arrangement is notcompletely satisfactory, particularly in connection with the leadingedge fillet.

[0004] This invention provides a unique solution to the above problem bydirecting cooling air through a tube, the forward end of which islocated closely adjacent the underside of the platform, below theforward portion of the pressure side of the airfoil, so as toeffectively impingement cool the platform in the localized region belowthe leading edge fillet.

[0005] In addition, film cooling holes are formed in the platform itselfso that air introduced into the cavity by the impingement cooling tubecan escape the cavity and flow along the upper surface of the platformalong the contour of the pressure side of the airfoil, in a directiontoward the trailing edge of the airfoil.

[0006] In its broader aspects, the invention relates to a turbine buckethaving an airfoil portion and a root portion, with a substantiallyplanar platform at an interface between the airfoil portion and rootportion, a platform cooling arrangement comprising at least one bore inthe root portion and at least one impingement cooling tube seated in thebore, the tube extending beyond the bore with an outlet in closeproximity to a targeted area on an underside of the platform.

[0007] In another aspect, the invention relates to a method of cooling aleading edge fillet region of a turbine bucket having an airfoil portionand a root portion, the airfoil portion joined to a platform extendingover the root portion, comprising: a) providing a through bore in aleading side of a cover plate in the root portion, the borecommunicating with a recess in the root portion underlying the airfoilportion; b) inserting a tube in the bore, with a tip of the tubeextending beyond the bore and in close proximity to a targeted area onan underside of the platform; c) supplying cooling air to the recess viathe tube such that the targeted area is impingement cooled by thecooling air passing through the tube.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]FIG. 1 is a partial side cross section of a bucket in accordancewith the invention;

[0009]FIG. 2 is a partial end view of a lower dovetail portion (of theleading side edge) of the bucket not entirely shown in FIG. 1; and

[0010]FIG. 3 is a top plan view of the bucket in accordance with theinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0011] With reference to FIGS. 1-3, a turbine bucket 10 includes anairfoil portion 12 extending vertically upwardly from a horizontal,substantially planar platform 14. The airfoil portion has a leading edge15 and a trailing edge 17. Below the platform 14, there are two pair ofso-called “angel wings” 16, 18 extending in opposite directions from theleading and trailing sides 20, 22 of the cover plate 24 in the rootportion of the bucket. The platform 14 is joined with the cover plate24. Below the root portion, there is a dovetail 26 by which the bucketis secured to a turbine wheel (in a preferred embodiment, the stage 1 orstage 2 wheels of a gas turbine).

[0012] Cooling passages (not shown) extend in radially outward andinward directions in a serpentine path through the airfoil portion, thepath having passages in the root portion communicating with supply andreturn passages 28, 30 in the dovetail. The internal airfoil coolingcircuit forms no part of the invention, however, and is therefore notshown in detail. Of significance here is the substantially hollow spaceor recess 32 in the root portion, above the dovetail 26 but below theplatform 14, defined further by the sides 20, 22 of the cover plate.This recess is generally on the pressure side 34 of the airfoil, and itwill be appreciated from FIG. 3 that a relatively large portion of theplatform 14 on the pressure side 34 of the airfoil overlies the hollowspace or recess 32.

[0013] An angled hole 36 is drilled through the leading side 20 of thecover plate 24 from the lower end of the root portion, on the externalside thereof, extending upwardly (i.e., substantially radially) andopening into the recess 32. Due to the angled orientation, the hole 36has an exit profile on the interior of the root wall 38 which iselliptical in shape as shown in FIG. 2. An impingement cooling tube 40is pressed into the hole 36 from under the root portion, so that anoutlet at the upper end or tip 42 of the tube is located close to theunderside of the platform, and close to a targeted area along theleading edge fillet 44 where the airfoil joins the platform.

[0014] The lower end of the tube 40 is flared at 46 to limit the extentof insertion, and the tube may be fixed in place by a spot weld at 47 toprevent the tube from sliding back through the hole in a directionopposite the assembly direction. Preferably, the tube end 42 is withinabout 0.5″ from the underside of the platform.

[0015] Since the lower edge of the cover plate 24 will seat on the wheelsurface, thus blocking the hole 36, a second hole 48 is drilled from theleading side 20 of the cover plate 24 so as to intersect with the hole36, just below the lower end of the tube 40. In this way, rotor or wheelspace purge air can enter the tube 40 via hole 36.

[0016] In addition, an array of film cooling holes 50 extend through theplatform 14 from the recess 32 to a location on the external side of theplatform.

[0017] With the above arrangement, purge air from wheel space below thebucket will enter the hole 48 and flow through the tube 40 so as toimpinge directly on the underside of the platform 14, below butproximate the leading edge fillet 44. The underside of the platform 14is thus impingement cooled in the leading edge fillet area to reducestress and improve low cycle fatigue margins. Of course, the enhancementof heat transfer in this specific region continues along the platformand at least partially under the airfoil itself.

[0018] Another feature of the invention lies in the combination of theabove described impingement cooling with film cooling along the uppersurface of the platform. This is achieved by utilizing the array of filmcooling holes 50 in the platform, allowing the cooling air in the hollowspace 32 to exit along the top surface of the platform. The film coolingholes 50 are directionally oriented so that the cooling air merges withthe gas path air and flows along the pressure side of the airfoil towardthe trailing edge 17. The flowpath of the film cooling air is such thatit can carry over to the suction side of the platform of the adjacentbucket, increasing the overall effectiveness of the system.

[0019] To further enhance heat transfer, the underside of the platform14 can be provided with any suitable rough coat, thus increasing thesurface area available for heat transfer. In addition, one or moreadditional impingement cooling tubes may be used in conjunction with thetube 40 to enhance heat transfer at targeted locations along theunderside of the platform.

[0020] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. In a turbine bucket having an airfoil portion anda root portion, with a substantially planar platform at an interfacebetween the airfoil portion and root portion, a platform coolingarrangement comprising: at least one bore in said root portion and atleast one impingement cooling tube seated in said at least one bore,said tube extending beyond said bore with an outlet in close proximityto a targeted area on an underside of said platform.
 2. The bucket ofclaim 1 wherein said targeted area lies within a recess under saidplatform, and proximate a leading edge fillet of said airfoil portion.3. The bucket of claim 2 wherein said platform is provided with aplurality of film cooling holes communicating with said recess.
 4. Thebucket of claim 3 wherein said plurality of film cooling holes arelocated closer to a trailing edge of said airfoil portion than to theleading edge of said airfoil portion.
 5. The bucket of claim 1 whereinthe underside of said platform is coated with a rough coat material. 6.The bucket of claim 4 wherein said impingement cooling tube and saidplurality of film cooling holes are located on a pressure side of saidairfoil portion.
 7. The bucket of claim 3 wherein said film coolingholes are directionally oriented to cause cooling air to flow in a pathgenerally toward said trailing edge of said airfoil portion.
 8. Thebucket of claim 1 wherein said impingement cooling tube is flared at itsrearward end to limit forward insertion into said bore.
 9. The bucket ofclaim 1 wherein said impingement cooling tube is secured within saidbore so as to prevent movement in a direction away from said platform.10. In a turbine bucket having an airfoil portion and a root portion,with a substantially planar platform at an interface between the airfoilportion and root portion, a platform cooling arrangement comprising:means for impingement cooling a targeted area on an underside of saidplatform.
 11. A method of cooling a leading edge fillet region of aturbine bucket having an airfoil portion and a root portion, saidairfoil portion joined to a platform extending over said root portion,comprising: a) providing a through bore in a leading side of a coverplate in said root portion, said bore communicating with a recess insaid root portion underlying said airfoil portion; b) inserting a tubein said bore, with a tip of the tube extending beyond said bore and inclose proximity to a targeted area on an underside of said platform; c)supplying cooling air to said recess via said tube such that saidtargeted area is impingement cooled by said cooling air passing throughsaid tube.
 12. The method of claim 11 wherein said targeted area is anarea in said recess substantially directly beneath the leading edgefillet of said airfoil portion.
 13. The method of claim 11 includingproviding film cooling holes to allow said cooling air to escape fromsaid recess.
 14. The method of claim 11 wherein said film cooling holesare located in said platform.
 15. The method of claim 14 wherein saidimpingement cooling tube and said film cooling holes are on a pressureside of said airfoil portion.
 16. The method of claim 15 wherein saidfilm cooling holes are directionally oriented to cause cooling air toflow in a path generally toward said trailing edge of said airfoilportion.